1. Field of the Invention
The present invention relates to a protective material protecting the surface of a photoreceptor used for electrophotographic image forming and an image forming apparatus using the protective material.
2. Discussion of the Background
In an electrophotographic image forming apparatus, an image bearer such as a photoconductive photoreceptors is subjected to a charging process, an irradiating process, a developing process and transferring process to form an image. Discharge products produced in the charging process, remaining on the surface of the photoreceptor and residual toners or toner components remaining thereon after the transferring process are removed in a cleaning process.
Conventional cleaning methods use an inexpensive and simple cleaning blade formed of a rubber or urethane, having good cleanability. However, since the cleaning blade is pressed to the surface of a photoreceptor to remove residues thereon, a stress due to friction between the surface of a photoreceptor and the cleaning blade is large and the cleaning blade and the photoreceptor, particularly an organic photoreceptor, are abraded, resulting in shorter lives thereof. In addition, a toner used for forming images is having a smaller particle diameter to produce higher quality images. The smaller the particle diameter, the more the toner scrapes through a cleaning blade. Particularly when the cleaning blade has insufficient dimensional accuracy, assemble accuracy or partially oscillates, the toner scrapes through the blade more, resulting in production of poor quality images.
So as to extend the life of an organic photoreceptor to produce high quality images for long periods, deterioration of members such as a cleaning blade due to abrasion needs to be reduced to improve cleanability thereof.
Japanese published examined application No. 51-22380 discloses a method of pulverizing a metal soap block such as zinc stearate by pressing a brush thereto to prepare a powder, applying the powder to a photoreceptor, and forming a film of a lubricant thereover with a cleaning blade.
The metal soap such as zinc stearate improves the lubricity of the surface of a photoreceptor to reduce the friction between the photoreceptor and the cleaning blade and improve the cleanability of an untransferred toner.
In the charging process, a DC voltage has been overlapped with an AC voltage to charge a photoreceptor with a charging roller (AC charge). This uniformly charges a photoreceptor, less produces oxidized gas such as ozone and NOx, and downsizes the apparatus. However, deterioration of the surface of a photoreceptor is accelerated by repeated discharges between a charger and the photoreceptor because positive and negative discharges therebetween repeat for several hundred to thousand times a second according to the frequency of the AC voltage applied to the photoreceptor. When a photoreceptor is coated with a lubricant, the charging energy is absorbed by the lubricant first and difficult to reach the photoreceptor, and which is protected.
The metal soap resolves with the energy, but does not completely resolve and disappear. A low-molecular-weight fatty acid is produced and friction between the photoreceptor and cleaning blade is likely to increase. A toner is likely to adhere to the photoreceptor in the form of a film with a fatty acid, resulting in deterioration of image resolution, abrasion of the photoreceptor and uneven image density.
Therefore, it is necessary to apply a large amount of the metal soap onto a photoreceptor to cover the surface thereof with the metal soap instantly even when the fatty acid is produced. Further, the linear speed of a photoreceptor increases to meet demands for forming images at higher speed, and the amount of the metal soap applied thereto needs increasing accordingly.
The metal soap block for use in image forming apparatus is typically prepared by casting melted metal soap into a mold and cooling as disclosed in Japanese published unexamined application No. 10-279998. Since the thus prepared metal soap block has an isotropic and precise crystal, the durability of a brush is not sufficient because of being pressed to the metal soap at higher pressure to apply a large amount of the metal soap to a photoreceptor having a high linear speed.
The particulate metal soap scraped by the brush is an amorphous granulated fine powder. The particulate metal soap is dammed, pulverized and coated on a photoreceptor by the blade while the linear speed thereof is low. However, when the metal soap is applied much and the linear speed of a photoreceptor is high, comparatively a large particulate metal soap passes the blade and reached the charging roller. The particulate metal soap electrostatically adheres to the charging roller, and is oxidized and melted with the charging energy, and finally fixed thereon. When the metal soap is fixed on the charging roller, the metal soap involves a toner present on a photoreceptor and a part of the charging roller the metal soap is fixed on has high resistivity, resulting in defective charging and production of images having black stripes.
Japanese published unexamined application No. 2000-319224 discloses a method of casting a metal soap into a mold heated to have a temperature lower than a melting point of the metal soap by 25 to 45° C. and compacting the metal soap under reduced pressure to prepare a metal soap block without crack and defect. This method covers an energy for a temperature lower than the melting point with a compression energy and eliminates an airspace in the metal soap block with depressurization to prepare a metal soap block which is almost the same one prepared by melting. However, when the metal soap is applied much and the linear speed of a photoreceptor is high, comparatively a large particulate metal soap passes the blade and reached the charging roller. The particulate metal soap electrostatically adheres to the charging roller, and is oxidized and melted with the charging energy, and finally fixed thereon. When the metal soap is fixed on the charging roller, the metal soap involves a toner present on a photoreceptor and a part of the charging roller the metal soap is fixed on has high resistivity, resulting in defective charging and production of images having black stripes.
In order to solve this problem in coating the metal soap on a photoreceptor, many suggestions are made.
Japanese published unexamined application No. 2007-140391 discloses a method of coating a metal soap with a coating roller on a photoreceptor and evening the metal soap thereon with a leveling blade having a specific or more hardness. not less than a specific hardness.
However, as shown in FIG. 4 in Japanese published unexamined application No. 2007-140391, particulate metal soaps having various sizes are present. Therefore, the leveling blade needs to have comparatively a high hardness to form a uniform lubricant layer and a pressure of the brush is precisely controlled to uniformly scrape the metal soap.
Namely, when the amount of the metal soap applied to a photoreceptor varies, particularly when it decreases, the blade having high hardness gives a large stress thereto, resulting in scratches thereof and abrasion of the blade.
Japanese published unexamined application No. 2006-84878 a method of contacting a flicking member to a brush when scraping and applying a metal soap block with a brush-shaped application member to surface of a photoreceptor to uniformly apply the metal soap thereto.
However, the scraped metal soaps still have various sizes and are not necessarily applied to a photoreceptor in the width direction.
Japanese published unexamined applications Nos. 2002-6679, 9-90847 and 9-138622 disclose methods of stabilizing an amount of the metal soap block applying to a photoreceptor. However, since additional members such as a metal soap oscillator, a cutter and a thermodetector are needed to stabilize an amount of the metal soap block applying to a photoreceptor, the mechanism and control are complicated.
Japanese published unexamined application No. 2000-338819 discloses a method of stabilizing an amount of the metal soap block applying to a photoreceptor over time. However, this does not consider at all the size of the scraped particulate metal soap and resolve positional unevenness of the amount of the metal soap block applying thereto.
Because of these reasons, a need exists for a protective material block protecting a photoreceptor even when rotating at high speed and capable of producing high-quality images without black stripes due to high resistivity of a charging roller.